Refractive indices of optical materials of glass materials, organic resins and the like generally become gradually higher with shorter wavelengths. Indices indicating wavelength dispersions of the refractive indices include Abbe number (νd) and the secondary dispersion characteristic (θg, F). The Abbe number (νd) and secondary dispersion characteristic (θg, F) are values unique to an optical material, but fall into certain ranges in many cases. Refractive indices and Abbe number (νd) of conventional optical materials are shown in FIG. 1.
The Abbe number (νd) and the secondary dispersion characteristic (θg, F) are represented by the following expressions.Abbe number[νd]=(nd−1)/(nF−nC)Secondary dispersion characteristic[θg,F]=(ng−nF)/(nF−nC)wherein nd is a refractive index at a wavelength of 587.6 nm; nF is a refractive index at a wavelength of 486.1 nm; nC is a refractive index at a wavelength of 656.3 nm; and ng is a refractive index at a wavelength of 435.8 nm.
However, by in detail designing constitutions (material types and molecular structures) of optical materials (glass materials, organic resins and the like), optical materials having a high secondary dispersion characteristic (θg, F) apart from the above-mentioned certain range are synthesized. For example, a polyvinylcarbazole as an organic resin (A in FIG. 1) of which secondary dispersion characteristic (θg, F) is higher than general-purpose organic resin materials.
In refractive optical systems, the chromatic aberration is generally reduced by combining glass materials having different dispersion characteristics. For example, in objective lenses of telescopes and the like, a glass material having a small dispersion is used as a plus lens, and a glass material having a large dispersion is used as a minus lens; and by combining these, the chromatic aberration appearing on the axis is corrected. Therefore, in the case where constitutions and the number of lenses are limited, in the case where a glass material to be used is limited, and in other cases, it is sometimes very difficult to sufficiently correct the chromatic aberration. As one method of solving such a problem, there is carried out designing of optical elements by utilizing glass materials having anomalous dispersion characteristics.
In the case of producing optical elements having an aspherical shape or the like excellent in the chromatic aberration correction function, the formation of an organic resin on a spherical glass or the like has better advantages in productivity, moldability, versatility in shape and weight reduction than the use of a glass material as a material. However, optical characteristics of conventional organic resins fall into certain limited ranges as illustrated in FIG. 1, and there are thus very few organic resins exhibiting specific dispersion characteristics.
Japanese Patent Application Laid-Open No. 2008-158361 describes an optical resin composition in which N-acryloylcarbazole, a polyfunctional polyester acylate, dimethyloltricyclodecane diacrylate, and a polymerization initiator are mixed in predetermined ratios. It describes that the optical resin composition is a material which is easily processed and whose cured product has sufficient anomalous dispersion and durability.
On the other hand, the present inventor has paid attention to that, in order to impart a higher chromatic aberration correction function to optical elements than conventionally, material with higher secondary dispersion characteristic (θg, F) is very effective on optical designing. Specifically, in FIG. 1, the characteristic is one within a range B (νd<25 and θg, F>0.73) where the relation between νd and θg, F is apart from plots of glass materials or general-purpose materials of organic resins.
However, there now exists no material having secondary dispersion characteristic (θg, F) within the range B in FIG. 1 and exhibiting little coloration and capable of being stably produced. The materials described in Japanese Patent Application Laid-Open No. 2008-158361 all have a θg, F value of 0.70 or lower.